Spark plugs deliver an electric spark into the combustion chamber of a spark-ignited piston engine. The internal combustion engine marketplace is froth with different types of spark plug configurations to serve a variety of functions. However, the spark plugs designed for piston-engine aircraft are particularly challenging due to the fact that bore sizes of the cylinder are generally larger (calling for 18 mm spark plugs) and each cylinder often utilizes 2 spark plugs, typically in a horizontally-opposed configuration, see FIG. 1.
Aviation spark plugs have a number of important attributes. For example, the barrel sizes vary between Size E—shielded ⅝ in. with 24 threads, and Size H—shielded ¾ in. with 20 threads. Aircraft mounting threads (18 mm)—include the following: Size B—with 13/16 in. reach and ⅞ in. hex; Size M—with ½ in. reach and ⅞ in. hex; and Size U—with 1⅛ in. reach and ⅞ in. hex. By comparison, automotive mounting threads (14 mm) have different sizes: Size J—with ⅜ in. reach and 13/16 in. hex; Size L—with ½ in. reach and 13/16 hex; and Size N—with ¾ in. reach and 13/16 hex.
The electrode design of a spark plug typically uses a conventional single center electrode with variations of one, two, three, four or more ground electrodes on a single plug. There are different design features (fine-wire, iridium, nickel, etc.) to evoke different sparking characteristics.
There have been hundreds of publications, periodicals and patent applications dealing with spark plug design and manufacture for use in automotive engines (e.g., Heywood, John. Internal Combustion Engine Fundamentals. McGraw-Hill, 1988 and Schwaller, Anthony, Motor Automotive Mechanics. Delmar Publishers, 1988). Notable among the patent field are those that reference the suppression of radio-frequency electromagnetic interference (e.g. U.S. Pat. Nos. 4,713,582 and 4,568,855) and the use of unique electrode designs (e.g., U.S. Pat. Nos. 6,091,185, 7,309,951 and 7,528,534) that offer more chances for the electric impulse in the piston engine to spark with resistance to fouling. However, none of the references are targeted at the unique challenges of the aircraft piston engine, which has more complexity and dimensional aspects that nullify inventions of the past.
Internal combustion engines in piston aircraft differ greatly from those in automobiles. Automobiles utilize a high rpm transmission with a gear reduction system, where piston aircraft do not have a transmission but instead have a much larger crankshaft and thrust bearings to directly rotate the propeller. As a result, aircraft cylinders are larger and the rpms are lower for aircraft engines.
Automobiles utilize water-cooled cylinders which are maintained at a constant temperature for stable operation, whereas piston aircraft cylinders are air-cooled by the inflow of outside air controlled by the pilot's throttle and airspeed. Detonation will occur in the aircraft engine when the cylinder gets too hot, which can be impacted by high outside air temperature and/or slow speeds at too high a deck angle. Certain pilot operating conditions may not lend themselves to lowering the angle of ascent, which is why either cooling the inlet air, cooling the cylinder, or increasing the octane of the fuel is critical to prevent detonation. Accordingly, many automotive spark plugs do not perform to the requirements of an aircraft engine.
It is also noteworthy that automobile engines are now highly automated whereby the air-to-fuel ratio is maintained at a constant level, adjusted for octane. By comparison, piston aircraft are operated manually at rich and lean mixture configurations subject to pilot discretion. This fact contributes greatly to the existence of combustion fouling from carbon, lead, etc. in aircraft engines when the fuel mixture is momentarily too rich and forms unwanted deposits on spark plugs.
Automobiles are generally operated up to about 30% of their rated power, whereas piston aircraft are generally operated above 75% of their rated power. This infers that piston aircraft are much more vulnerable to detonation incidents because full power is needed at take-off, while cross-country cruise is generally at about 75% power. Accordingly, there are few options to safely lessen the load on the aircraft engine at full power during take-off to avoid detonation. Having a clean spark and unfouled plugs becomes a vital safety issue in an aircraft.
Automobiles use smaller spark plugs with a typical bore size of 2″ to 4″, while most piston aircraft use larger horizontally-opposed spark plugs (2 in each cylinder) with bore sizes between 3″ to 6″. Automobiles have engine rotation speeds ranging from 0-7,000 rpm but rarely operate above ⅓ the maximum rpm available. However, piston aircraft typically have a maximum rotation up to about 2,800 rpm and often operate at or near this maximum a high percentage of the time while in flight. This high rpm activity in propeller aircraft is intensified by the electronic pulse of the piston which can cause electromagnetic interference which can disrupt pilot radio signals and navigational systems—creating a dangerous condition in flight.
In the last several decades the compression ratio of most automotive engines, measuring the ratio of the max vs. min volume in the cylinder has ranged between 9:1 to as high as 14:1. Such ratios on high performance aircraft are lower, typically ranging between 7.5:1 up to 9:1 (with naturally aspirated engines having ratios the higher end and turbocharged engines at the lower end.)
All these factors and more impact the way fuel is combusted and pre-mature engine detonation (knock) is controlled. This is particularly the case when adding the complexity in aircraft at high altitudes needing low vapor pressure gasoline with very high octane levels to sustain peak performance.